With the ever-increasing awareness of needlestick injury and the risks associated with sharing needles, there have been many syringes introduced in the marketplace having some form of retractable needle. There are three basic types.
The first type can be seen as a “manual”-type syringe where, as the plunger is moved forwardly towards the end of the syringe barrel, it couples with a needle holder which contains the needle. The plunger can then be manually retracted which causes the needle to be retracted into the syringe body.
The second type can broadly be classified as “shoot back” syringes. In these syringes, some form of spring is provided. As the plunger is pushed forwardly towards the end of the syringe barrel, the needle holder is released and the spring causes the needle holder (containing the attached needle) to shoot back, either into the plunger body or into the syringe body. The spring may be mounted about the needle holder and under permanent compression until the needle holder is released. Alternatively, it is known to provide a spring that stretches as the plunger is pushed forwardly towards the front of the syringe barrel.
The third type can be broadly classified as “suck back” syringes. In these syringes, a reduced pressure (vacuum) is typically provided in the plunger and the front of the plunger is sealed by a piston. The piston is releasably mounted to the front of the plunger. The front of the syringe barrel has a releasable needle holder which contains the needle. As the plunger is pushed forwardly towards the end of the syringe barrel, the piston couples to the needle holder and at the same time, the piston is released from the front of the plunger and the needle holder is released from the front of the syringe barrel which causes the needle holder/piston to be sucked back into the plunger by the vacuum.
There are many variations to these three basic constructions. For instance, it is known to provide a “vacuum on demand” to the plunger which means that the syringe can be at atmospheric pressure until just before use at which stage a vacuum can be created in the plunger.
In each of the basic constructions, great care needs to be taken that the plunger (which may include a piston) properly couples to the needle holder such that the needle holder (containing the contaminated needle) can be properly retracted.
With a manual retraction, it is undesirable that a situation can occur where retraction of the plunger does not cause retraction of the needle holder.
With a shoot back mechanism, it is highly undesirable that the mechanism “triggers” either too soon or too late. If the mechanism triggers too soon, there may still be appreciable medicine in the syringe which will be lost if the needle shoots back prematurely. Alternatively, it is also highly undesirable if the mechanism does not trigger when the plunger is pushed fully forwardly, or where it is necessary to place undue force on the plunger (which can damage the shoot back mechanism).
With a “suck back” mechanism, it is also highly undesirable that the mechanism triggers too soon or too late. With this type of mechanism, because the plunger is under vacuum, if the piston on the front of the plunger releases too early, the piston can be retracted (sucked back) into the plunger body without properly attaching to the needle holder. Alternatively, it is equally undesirable that the needle holder is released too early.
However, it is found that the particular construction of the single use syringes is such that there is not much “leeway” in the triggering mechanism, and particularly if the syringes are the small 1 ml syringes. Thus, the possibility of the release mechanism triggering either too late or too soon is ever present.
One cause for the premature triggering of the release mechanism seems to be that the plunger needs to engage with the retraction device almost immediately upon the plunger being pushed into a forward part of the syringe. Thus, if the engagement does not occur immediately, there may be malfunctioning of the release mechanism.
Another possible cause for premature triggering or delayed triggering may be in the way that the needle holder (often also called a luer) is attached in a releasable manner to the front of the barrel. It is known to hold the needle holder in a releasable manner to the front of the barrel using some sort of functional engagement, and when the plunger is pushed towards the front of the barrel, the plunger engages in some manner with the needle holder to reduce to frictional engagement to such an extent that the needle holder can now be triggered to the retracted position. It is found that a frictional engagement of the needle holder to the barrel is not necessarily an entirely satisfactory arrangement.
Therefore, it is also known to provide some form of step or shoulder in the front of the barrel against which the needle holder can be held and it is then necessary to push the step or shoulder away to release the needle holder. It is also known to provide some form of “shatter plate” which is broken to release the needle holder, or some form of frangible portion. While some of these arrangements can provide a satisfactory temporary attachment of the needle holder in place, the manufacture of the step or shoulder in the front of the barrel can be quite difficult.
It would therefore be an advantage to modify the particular construction of single use syringes, including of the “suck back” type to improve the design by which the piston can be released from the front of the plunger and/or the design by which the needle holder can be released from the front of the syringe barrel.
It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.